Don't reinstall perl.

See #110. [skip ci]

.appveyor.yml

@@ -8,7 +8,7 @@ install:
     - curl -ostack.zip -LsS --insecure https://www.stackage.org/stack/windows-x86_64
     - 7z x stack.zip stack.exe
     - bash -lc "curl -LsS http://www.haskell.org/ghc/dist/7.10.3/ghc-7.10.3-x86_64-unknown-mingw32.tar.xz | tar -xJ -C /mingw64 --strip-components=1"
-    - stack exec -- pacman -S --noconfirm perl gcc binutils p7zip git
+    - stack exec -- pacman -S --noconfirm gcc binutils p7zip git
     - echo "" | stack --no-terminal install alex happy shake ansi-terminal mtl
     - git clone --recursive git://git.haskell.org/ghc.git C:\msys64\home\ghc\tmp
     - bash -lc "mv /home/ghc/tmp/* /home/ghc"